1. Field of the Invention
The present invention relates in general to predicting the level of noise or brake squeal produced by a brake part during braking, and relates in particular to a method and apparatus for defining and quantifying vibration and noise suppression of a brake part as a single number known as the quality factor or Q-factor.
2. Description of Prior Development
A longstanding problem associated with the use of vehicle brakes is the generation of annoying noise often referred to as brake squeal. Brake rotors and drums, such as used in virtually all transportation vehicles, are generally considered to be the source of a variety of such noises and associated vibrations.
In order to reduce brake noise, brake rotors and drums have been manufactured using materials and processes which tend to reduce the vibrations produced during braking. Typically, the greater the ability of a brake part to damp vibrations, the less apt the part is to make undesirable braking noise.
Brake component manufacturers, as well as brake system designers, manufacturers and assemblers have attempted to control brake system noise by specifying a minimum amount or minimum level of vibration damping inherent in brake components such as brake rotors and brake drums. Unfortunately, conventional vibration damping measuring techniques used to establish and verify vibration damping properties of brake components have not always provided consistent measurements.
That is, prior vibration measuring techniques attempted to qualify vibration damping performance in terms of a quality factor commonly expressed as a dimensionless number referred to as the Q-factor. The lower the Q-factor, the higher the damping performance of the part and the less likely the part is to squeal during use. In order to quantify the amount of vibration damping in a part, the Q-factor is calculated from the product of a constant term of 27.3 multiplied by the resonant frequency of the part in Hertz and divided by the slope of the vibration decay curve of the part.
Although Q-factors have been specified by original equipment manufacturers (OEMs) such as car and truck manufacturers, significant differences in Q-factors have been measured by suppliers and OEMs when measuring the same components. Variations in Q-factor measurement of up to 25 percent have been experienced, even when measuring the same part several times in the same test fixture.
This lack of repeatability in Q-factor measurement can lead to acceptance issues between manufacturers and suppliers regarding the acceptability of brake components.
U.S. Pat. No. 6,014,899 discloses a method of determining the Q-factor, wherein vibration decay rate measurements are taken at a plurality of spaced locations along the tested part. The Q-factors obtained at the various test locations can be averaged to obtain a representative Q-factor for the test part. One difficulty with the method described in U.S. Pat. No. 6,014,899 is that considerable time is required to carry out the measurements at all of the test locations. Typically, decay rate measurements are required at twenty or more locations on the tested part. For optimum results, plural measurements are taken at each test location.
Copending patent application Ser. No. 09/519,485 discloses a method for determining optimum locations on the part for performing the decay rate measurements. Use of this method can reduce the number of decay rate measurements required to determine a representative Q-factor.
In the methods described in U.S. Pat. No. 6,014,899 and U.S. patent application Ser. No. 09/519,485 the tested part is put into the vibrational mode by means of an exciter coil that is supplied with a sinusoidal alternating current having a D.C. offset. The tested part vibrates at the same frequency as the A.C. current supplied to the exciter coil.
The described method of coil excitation has some disadvantages. The output of the sine wave generator must be amplified accurately in order to produce a strong enough force to move a part at the desired frequency and amplitude; typically the tested part weights on the order of ten pounds. The amplifier needs to be able to amplify both the sine wave and the D.C. Voltage level linearly, with an equal gain for both the A.C. and D.C. portions over the frequency range of interest. Ripples or variations in the D.C. current can have an adverse effect on the repeatability (or accuracy) of the amplitude in the A.C. current waveform supplied to the exciter coil. The trough of the amplified sine wave must have a repeatable non-symmetrical relationship to the zero current axis in order to achieve a satisfactory vibrational amplitude in the test part.
The described arrangement requires a relatively expensive amplifier capable of accurately amplifying the D.C. and A.C. portions of the signal. Heat dissipation and non-linearity problems associated with creating the D.C. input signal have to be considered.
The present invention relates to a method of coil excitation wherein an A.C. waveform is employed at one half the frequency of the vibration frequency imparted to the tested part. The A.C. waveform is symmetrical around the zero current axis, so that the A.C. current crosses the zero current axis at twice the A.C. current frequency. The magnetic flux in the coil reverses polarity in synchronism with the A.C. current zero-crossover occurrences, such that the part vibrates at twice the frequency of the A.C. waveform supplied to the exciter coil.
A principal advantage of the invention is that the coil excitation current requires no D.C. offset current. A relatively low cost amplifier can be used to supply the coil with an A.C. waveform that is repeatable, without uncertainties as to amplifier performance. The amplifier is required only to amplify a pure A.C. sine wave; no D.C. offset current is used.
Further features of the invention will be apparent from the attached drawings and description of an apparatus used in practicing the method of the present invention.